Electrokinetic separation of solid particles from aqueous suspensions thereof

ABSTRACT

A process for the electrokinetic separation of finely divided clay particles from an aqueous suspension thereof, the suspension being contained in an electrokinetic cell equipped with an anode and cathode separated by a semi-permeable membrane impermeable to clay particles. Upon activation of the cell with direct current, clay particles are caused to deposit electrophoretically on the anode. Simultaneously with the deposition of the clay deposit, portions of the water contained in the suspension and entrained in the deposit are caused to migrate electroosmotically through the membrane and collected at a cathode compartment.

REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 436,408,filed Jan. 25, 1974 now abandoned, and is a continuation-in-part ofcopending application Ser. No. 403,191 filed Sept. 26, 1973 nowabandoned, and of its parent application Ser. No. 263,376 filed June 5,1972 now abandoned, in the name of Albert C. Kunkle.

BACKGROUND OF THE INVENTION

This invention relates to a process for the separation of finely dividedclay particles from aqueous suspensions thereof and, more particularly,to an electrokinetic process wherein the aqueous suspension is effectedsimultaneously by electrophoresis and electroosmosis.

Clay such as kaolin, bentonite and the like are widely used asadsorbents, pigments, catalysts, paper fillers and the like. These claysgenerally occur in geological deposits as mixtures with inert foreignmaterials, and it is necessary to separate the clay material before theclay is utilized in commercial applications.

In effecting the separation, the crude clay is dispersed in water withthe aid of dispersing chemicals such as sodium silicate, sodiumhexametaphosphate, sodium tripolyshosphate and tetrasodium pyrophosphateand the clay slurry is blunged, degritted, classified and leached toeffect removal of the undesired foreign materials. Following thesesteps, the clay is filtered to remove the dispersing and leachingchemicals and to produce a solid filter cake containing 50% to 60% clay.This 50% to 60% solids cake is then redispersed and spray dried orotherwise dried in the flocculated state. For various economic reasonsit is desirable in the clay industry to ship slurries containing 70%solids. Therefore, it is conventional in the clay art to addapproximately 35% to 50% spray dried clay to a 50% to 60% solidsredispersed filter cake to yield a 70% solids slurry. The addition ofspray dried clay to clay filter cake to prepare a 70% solids slurry forshipment adds significantly to the high cost of the shipped slurry dueto the relatively high costs of preparing the spray dried clay. The art,therefore, has been continually seeking to effect methods whereby clayfilter cakes containing 70% clay can be directly obtained from claysuspensions without the addition of spray dried clay.

Among the methods which the art has investigated it its attempt toobtain a more concentrated, i.e., 70% solids clay cake, has been the useof electrokinetic phenomenon such as electrophoresis and electroosmosis.

Most solid materials when suspended as fine particles in a liquidacquire an electric charge. By applying a DC (Direct Current) electricfield between two electrodes immersed in the particle suspension, theparticles are caused to travel toward one of the electrodes and form adeposit thereon. This travelling of solid particles through a liquid dueto the application of DC is referred to in the art as electrophoresis.When under the influence of a direct current potential, water or otherliquid medium is caused to migrate through a stationary porous diaphragmtoward a charged electrode. This phenomenon is referred to in the art aselectroosmosis. Both electrophoresis and electroosmosis have beenapplied to the separation of clays from aqueous suspensions thereof.When applied to aqueous clay suspensions, electrophoresis is generallyused to effect the deposition of the suspended clay material on acharged electrode, whereas electroosmosis functions as an aid inconsolidating and concentrating the electrophoretically deposited clayby removal of the entrained water from the deposit.

General descriptions of these phenomena may be found at Kirk-Othmer,Encyclopedia of Chemical Technology, V. 5 (1950, pp. 549-551, 606-610,V. 7, (2d Ed. 1965) pp. 841-865, V. 8, (2d Ed. 1965) pp. 23-36,Interscience Encyclopedia, Inc., N.Y., N.Y.; Poole-Doyle, Solid-LiquidSeparation, pp. 44-59, 100-188, 471-479, 511-513, 627-634, 698, 749,750, 782, 835-844, 869, 873, 922, 923, Monlik, "Physical Aspects ofElectrofiltration", Environmental Science & Technology, V. 5, No. 9,September 1971, pp. 771-776; Reif, "Electrokinetics", IndustrialResearch, December 1971; Creighton, Electrochemistry, V. 1, pp. 143-165,John Wiley & Sons, Inc., N.Y., N.Y. 1943; Sennett-Olivier, "ColloidalDispersions, Electrokinetic Effects, and the Concept of Zeta Potential"Chemistry and Physics of Interfaces, pp. 73-92 American ChemicalSociety, Washington, D.C., 1965; Daniels-Alberty, Physical Chemistry,pp. 512-516, John Wiley & Sons, Inc., N.Y., N.Y., 1955; Glasstone,Textbook of Physical Chemistry, 2d Ed., pp. 1219-1240, D. Van NostrandCo., Inc., Princeton, N.J., 1946.

Descriptions of some specific applications may be found in Curtis, C.E., "The Electrical Dewatering of Clay Suspensions", J. Am. Ceram. Soc.,14, 219 (1931); Drever, J. I., "The Separation of Clay Minerals byContinuous Particle Electrophoresis", Am. Mineralogist, 54, 937 (1969);and Miller-Baker, "Electrophoretic-Specific Gravity Separation of Pyritefrom Coal", Report 7440, Bureau of Mines, Dept. of Interior, (1970).

In addition, the following patents are representative of the prior artsattempts to employ those phenomena in practical applications: Ser. No.936,805 (1963); U.S. Pat. Nos. 670,350; 720,186; 894,070; 972,029;993,888; 1,121,409; 1,133,967; 1,156,715; 1,174,946; 1,233,713;1,229,203; 1,235,063; 1,266,329; 1,326,106; 2,099,328; 2,236,861;2,295,476; 2,440,504; 2,448,848; 2,485,335; 2,500,878; 3,396,097;3,412,002; 3,412,008; 3,455,805; 3,497,439; 3,533,929; 3,556,969;3,589,991; and 3,616,453.

While the effect of electrophoresis and electroosmosis on clayseparation has long been known, they are not commercially successful bythe present day art. Attempts by the art to replace the filtrationprocedures conventionally used to prepare clay filter cakes from claysuspensions with electrophoretic or electroosmotic devices have not beennotably successful, particularly from an economic standpoint.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a method forelectrokinetically obtaining clay filter cakes having a clay solidscontent of at least 70% wherein an aqueous clay suspension having a claysolids content considerably less than 70% is introduced into anelectrokinetic cell provided with an anode and a cathode, the anode andcathode being separated by a semi-permeable membrane impermeable to thepassage of clay particles, the anode forming an anodic compartment withthe membrane and the cathode forming a cathode compartment with themembrane. The aqueous suspension containing the clay to be separatedtherefrom is passed into the anodic compartment and a direct currentfield is applied between the anode and cathode of the electrokineticcell causing the clay solids to be electrophoretically deposited on theanode while simultaneously causing the water of the slurry, and thewater entrained in the clay deposit, to migrate electroosmoticallythrough the semi-permeable membrane and into the cathode compartment.

As will hereinafter be more fully demonstrated by the practice of theprocess of the present invention, a high solids clay cake deposit havinga solids content in excess of 70% will be obtained at the anode, and theslurry solids in the effluent removed from the cell will remain constantthereby eliminating the need for additional treatment of the effluent.By the practice of the present invention, it is possible to effect theelectrophoretic deposition of clay at the anode of an electrokineticcell simultaneous with the electroosmotic dewatering of both thedeposited clay cake, and the effluent slurry remaining afterelectrokinetic activation of the clay suspension fed to the cell.

BRIEF DESCRIPTION OF THE DRAWINGS

Objects of the invention, together with additional features contributingthereto and advantages accruing therefrom, will be apparent from thefollowing description of two embodiments of the invention when read inconjunction with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-section representation of one embodiment ofan apparatus for use in the practice of the process of the presentinvention;

FIG. 2 is a schematic cross-sectional representation of a second andpreferred embodiment of an apparatus which may be used to practice theprocess of the present invention; and

FIG. 3 is a schematic cross-sectional detailed representation of thecathode structure preferred in the present invention.

PREFERRED EMBODIMENTS

Apparatus suitable for carrying out the electrokinetic process of thepresent invention is shown in copending application Ser. No. 436,414,filed concurrently herewith now abandoned and entitled, "ElectrokineticCell."

Briefly as described herein and as shown schematically in FIG. 1, theelectrokinetic cell 10 consists of a solid anode 11 and a foraminouscathode 12 separated by semi-permeable membrane 13. The anode andcathode are connected to a source of DC not shown. The semi-permeablemembrane 13 and the cell wall 14 form cathode compartment 15 containingthe cathode 12 therein.

In operation, the clay suspension 16, to be electrokineticallyseparated, is introduced into the cell 10 by a suitable liquid inletmeans 17 to the anode compartment 18 formed between the cell wall 19 andthe membrane 13. Upon activation of the cell with DC, a high solids cake20 of clay is electrophoretically deposited from the clay suspension 16onto the anode 11 while a portion of water is simultaneouslyelectroosmotically removed from the suspension 16 and the clay deposit20, and migrates through the membrane 13 and is collected in the cathodecompartment 15.

In FIGS. 2 and 3 the electrokinetic cell 22, as shown, consists of arotatably mounted, electrically conductive drum anode 23, the rotationthereof being effected by a source of power, not shown. Spaced apart,but concentric with the drum 23, is stationary, foraminous, arcuatecathode 29 which is mounted on and insulated from the upstanding wallsof tank 30. The anode 23 and cathode 29 are connected to a source of DCwhich is not shown. Spaced apart and separating the anode 23 and thecathode 29 is an arcuate semi-permeable diaphragm or membrane 31, alsomounted on and insulated from the tank walls and concentric with theanode 23 and cathode 29. The membrane 31 is flexible, tightly wovenporous fabric which is adapted to permit diffusion of water but isimpermeable to the passage of clay solids.

The space between the diaphragm 31 and the rotating drum 23 forms anddefines an anodic compartment 32 to which liquid inlet means 33 (anysuitable conduit or tube of dielectric, non-corrodable material, forexample, rubber or glass) extends from a suitable supply source (notshown) to the anodic chamber 32. Liquid outlet means 34 are alsoprovided to discharge electrokinetically treated clay suspension fromthe anodic chamber 32 and comprises a conduit or tube of a materialsimilar to that of the anodic chamber inlet (33) material.

The diaphragm 31 and the tank 30 define a cathodic compartment 35containing the foraminous cathode 29 spaced inwardly from the diaphragm31. The cathodic compartment 35 serves as a collection chamber for waterelectroosmotically separated from the clay slurry fed to the anodiccompartment as well as entrained water electroosmotically removed fromclay solids electrophoretically deposited on the drum. Electrolyte inletmeans 36 and electrolyte outlet means 37 are provided in the tank 30 toallow water or other electrolyte to be conducted to and from thecathodic compartment 35.

In operation, the clay suspension to be electrokinetically separated isintroduced by inlet means 33 into the anodic compartment causingportions of the drum 23 to become submerged in the clay suspension.Electrolyte solution is next admitted by electrolyte inlet means 36 intothe cathodic chamber 35 in amounts sufficient to submerge the cathode 29and contact the lower surface of the diaphragm 31. Upon activation ofthe rotating drum 23, the desired DC voltage is applied to the anode 23and cathode 29. Clay suspension is then circulated through the anodiccompartment 32. During the passage of the clay suspension in the anodiccompartment 32 beneath the rotating drum, and while it is confined tothe space between the anode 23 and the membrane 31, finely divided claysolids in the suspension are caused to be electrophoretically depositedon and adhere to the surface of the drum anode 23. The direction ofrotation of the drum 23 (indicated by the arrows in FIG. 2) advances theclay solids which have deposited on the drum surface during its passagethrough the anodic compartment 32 upwardly and over to the opposite sideof the drum where the deposit is thereupon removed from the drum surfaceby any removal means such as a scraper or string which contacts the drumat discharge point 28. Upon contact with the scraper or other removalmeans, the removed solids are discharged into a suitable clay cakecollection means, not shown.

The rotation of the drum as well as the flow of clay suspension admittedto the anodic compartment is continuous so a clean drum surface iscontinuously presented to the clay suspension introduced into theelectrokinetic cell.

Simultaneous with the electrophoretic deposition of the clay solids onthe drum, the water entrained in the clay cake immediately deposited onthe drum as well as a portion of the water in the anodic chamber 32 iscaused to migrate electroosmotically through the diaphragm 31 towardsthe cathode 29. Upon reaching the cathode 29, the waterelectroosmotically removed from the anodic chamber 32 percolates throughthe foraminous surface of the cathode and into the cathode compartment35 from which it is then removed from the cell. As the entry of waterinto the electrolyte in the cathodic compartment may cause undesireddilution of an electrolyte solution, fresh electrolyte solution must becontinuously circulated through the cathodic compartment 35 to maintainthe electrolyte concentration at the desired level.

As will hereinafter be demonstrated, due to simultaneous effect ofelectrophoresis and electroosmosis, clay filter cakes are obtainedhaving solids contents in excess of 70% and the solids contents of theeffluent suspension discharged from the electrophoretic cell aresubstantially equal to the solids content of the clay suspension chargedto the electrokinetic cell.

The electrolyte which is circulated through the cathode compartment ofthe electrokinetic cell is generally comprised of water having somemineral content or a dilute or weak acid such as sulfuric acid,hydrochloric acid or phosphoric acid. The acid solution generallyranging in strength from 0.1% to 1% is metered into the cathodecompartment at a rate sufficient to maintain the pH of the electrolytein the cathode compartment between 2.0 and 7.0.

It has been determined that the rate of deposition per unit power (i.e.,pounds per kilowatt-hour) increases when a weak solution of anelectrolyte (i.e., sulfuric acid) is circulated in lieu of water in thecathodic compartment of the electrokinetic cell.

In practicing the process of the present invention, the materials usedin the fabrication of electrodes of the electrokinetic cell may be anyof the well known corrosion resistant, conductive materials, metals andalloys, but for ease of maintenance the electrodes should be asresistant to chemical reaction with the clay suspension as is possible.Typical anode materials include antimony-lead alloy, platinum andconductive oxide coatings on tantalum or titanium and the like. Cathodematerials include the cathodic metals such as stainless steel andaluminum.

The semi-permeable membrane may be of any suitably permeable materialsuch as Dacron, nylon, polyesters, polypropylene having a porosity of0.5 to 4.0 cubic feet per minute.

As shown in greater detail in FIG. 3, a membrane preferred for use inthe practice of the present invention is composed of Dacron and isinsulated from the cathode 29 by an electrical insulator 40 such as aneoprene rubber spacer.

To achieve efficient operation of the electrokinetic cell, it isadvantageous that the spacing between the anode and the membrane be inthe order of about 1 inch to about 11/2 inches and preferably about 11/4inches. The spacing between the cathode and the membrane issignificantly less and is desirably in the order of 1/16 inch to 1/2inch and preferably about 3/16 inch.

In practicing the process of the present invention, electrokineticseparation of clay suspensions is advantageously achieved at a currentdensity of from about 0.05 to 0.35 amperes per square inch of effectiveelectrode area. At these current densities, the applied voltage willtypically range from 25 to 100 volts.

The process of the present invention is illustrated by the exampleswhich follow:

EXAMPLE I

To an electrokinetic cell of the type illustrated in FIG. 1 wasintroduced into the anode chamber 18 a central Georgia coating gradeclay having a particle size of 92% finer than 2 microns, dispersed tominimum viscosity at 60% solids with tetrasodium pyrophosphate andhaving a pH of 6.5. A 0.1% sulfuric acid was employed in the cathodiccompartment 15. The effective electrode area was 32 square inches. Thespacing between the anode 11 and membrane 13 was 11/4 inches and thespacing between the cathode 12 and membrane 13 was 1/4 inches. Thematerial from which the membrane 13 was formed was Dacron and had anaverage porosity of 1.5 cubic feet per minute. The anode 11 wasconstructed of lead, and the cathode 12 was a 100 mesh screen of 316stainless steel.

A series of runs were made delivering varying current densities andvoltages to the cell. The clay deposition period in each run was 5minutes. The deposition rate of the clay filter cake and the change inthe weight per unit power with varying current density (amps/sq. in.)are recorded in Table I below.

                                      TABLE I                                     __________________________________________________________________________    Current    Wet   Cake                                                                              Clay      Total Clay                                     Density                                                                             Voltage                                                                            Cake Wt.                                                                            Solids                                                                            Removed                                                                            Gms  Used                                           (amps/in.sup.2)                                                                     (volts)                                                                            (gms) (%) (%)  Kw-min                                                                             (gms)                                          __________________________________________________________________________    0.08  34   271   79  17   530  1260                                           0.11  42   351   79  20   360  1390                                           0.15  50   435   79  27   280  1270                                           0.19  57   520   79  31   244  1320                                           0.21  66   620   79  37   228  1320                                           0.25  75   697   78  40   186  1360                                           0.31  80   849   78  49   174  1350                                           0.37  95   963   78  55   121  1360                                           __________________________________________________________________________

The data in Table I indicate that the amount of clay deposited increaseswith increasing current density, but the clay deposited per unit powerdecreases. The solids of the electrophoretically deposited cake did notchange with current density and were on the order of 78-79%.

EXAMPLE II

The procedure of Example I was repeated with the exception that thecurrent density was maintained at 0.15 amperes per square inch and thetime of clay deposition was varied from 2 to 10 minutes. The effect oftime on the rate of deposition (grams per minute) and on cake solids isrecorded in Table II below.

For purposes of contrast, control runs were made repeating the procedureof Example II with the exception that a semi-permeable membrane was notemployed in the electrokinetic cell. The results of these control runsare also recorded in Table II.

                                      TABLE II                                    __________________________________________________________________________                                      Control                                         Total Wet                                                                            Gms Cake                                                                              Effluent  Clay Effluent                                    Time                                                                              Cake Wt.                                                                             Cake                                                                              Solids                                                                            Solids                                                                             Effluent                                                                           Removed                                                                            Solids                                                                             Effluent                               (min.)                                                                            (grs.) Min.                                                                              (%) (%)  pH   (%)  (%)  pH                                     __________________________________________________________________________    2   200    100 76  60   6.5  17   57   7.5                                    4   375    94  78  58   6.5  37   54   8.4                                    7   650    93  79  57   6.5  51   48   9.2                                    10  850    85  80  55   6.5  69   41   11.3                                   __________________________________________________________________________

The data in Table II indicates that the rate of clay depositiondecreases with increasing time. Cake solids increase with increasingtime of electrokinetic cell operation.

The data in Table II shows that by following the practice of the presentinvention there is obtained a filter cake having a solids content in theorder of 76%-80%, an effluent having a constant pH and a solids contentin the range of 55%-60%. By way of contrast in the control runs using anelectrophoretic process where simultaneous electroosmosis is absent, theeffluent solids range from 41%-57% and the effluent pH varies widelyfrom 7.5 to 11.3 pH.

EXAMPLE III

The procedure of Example I was repeated with the exception that theconcentration of clay in the slurry introduced into the electrokineticcell was varied from 20%-60%. Collection time and voltage weremaintained constant at 10 minutes and 50 volts. The results of thesetests are recorded in Table III below.

For purposes of contrast, control runs were made using the procedure ofExample III with the exception that the semi-permeable membrane was notused in the electrokinetic cell. The results of these control tests arealso recorded in Table III below.

                                      TABLE III                                   __________________________________________________________________________    Total                                                                         Wet                        Control                                            Feed                                                                              Cake                                                                              Cake                                                                              Effluent       Effluent                                           Solids                                                                            Wt. Solids                                                                            Solids                                                                             Effluent                                                                           Gms. Solids                                                                             Effluent                                                                           Gms.                                     (%) Gms.                                                                              (%) (%)  pH   Kw-min.                                                                            (%)  pH   Kw-min.                                  __________________________________________________________________________    60  1200                                                                              78  55   6.7  210  38.1 10.9 194                                      50  830 78  45   6.7  167  26.9 11.1 156                                      40  600 77  35   6.8  150  15.1 11.3 127                                      30  590 66  23   6.8  143  6.57 11.4  96                                      20  --  --  --   --   --   --   --   --                                       __________________________________________________________________________

A clay cake did not form on the anode when solids in the suspension fedto the electrokinetic cell were less than 20%. The data in Table IIIindicate that the clay deposition rate, cake solids, and dewateringefficiency decrease with decreasing feed solids.

It is readily apparent from the data in Table III that superior resultsin clay deposition, effluent solids, and effluent pH are obtained inaccordance with the present invention when these results are comparedwith the control where a semi-permeable membrane was not used in theelectrokinetic process.

EXAMPLE IV

To an electrokinetic cell of the type illustrated in FIG. 2 wasintroduced in the anode compartment 32 between the rotating anode 23 andthe membrane 31 a Central Georgia coating grade clay having a particlesize of 82% finer than 2 microns, having a 60%-61% solids concentrationand a pH of 6.7. The spacing between the anode 23 and the semi-permeablemembrane 31 was 11/4 inches and the spacing between the cathode 29 andthe membrane 31 was 3/16 inches. The semi-permeable membrane 31 was aDacron web having a porosity of 1.5 cubic feet per minute. Thedimensions of the anode 23 were 1 × 1.5 feet and the drum was rotated at0.16 rpm. The results of these runs are recorded in Table IV below.

                                      TABLE IV                                    __________________________________________________________________________    Current                                                                       Density                                                                            Slurry Effluent                                                                             Cake  Rate*                                                                              lbs.                                            (A/in)                                                                             Solids (%)                                                                              pH  Solids (%)                                                                          lbs/hr.                                                                            kw-hr.                                          __________________________________________________________________________    .13  59.6      6.7 79.2  50   42                                              .17  58.0      6.7 79.0  65   33                                              .19  58.0      6.7 79.0  76   34                                              .21  58.0      6.7 79.0  85   30                                              __________________________________________________________________________     *Dry Clay                                                                

EXAMPLE V

The procedure of Example IV was repeated with the exception that theclay slurry feed had a solids content of 60% and a pH of 6.5.

For the purposes of contrast, control runs were made using the procedureof Example V with the exception that the semi-permeable membrane 31 wasnot employed in the electrokinetic cell. The results of these controltests are also recorded in Table V below.

                  TABLE V                                                         ______________________________________                                                Cake  Effluent Effluent % Clay lbs.                                           Solids                                                                              Solids   pH       Removed                                                                              kw-hr.                                 ______________________________________                                        Present   79%     60%       6.7   65%    28.2                                 Invention                                                                     Control   79%     44%      10.5   65%    19.4                                 ______________________________________                                    

Table V shows that significant differences obtained with the presentinvention are higher effluent solids and lower power consumption per tonof clay. Thus the present invention eliminates the need for effluentrecovery and the rapid and extensive increase in effluent pH, whilestill providing a high solids cake.

While the invention has been described with reference to embodiments, itwill be understood by those skilled in the art that various changes maybe made and equivalents may be substituted for elements thereof withoutdeparting from the scope of the invention. In addition, manymodifications may be made to adapt a particular situation or material tothe teachings of the invention without departing from the essentialscope thereof. Therefore, it is intent that this invention not belimited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method of electrokinetically separating finelydivided clay particles suspended in an aqueous medium whichcomprisesintroducing a suspension of the solid particles into anelectrokinetic cell provided with an anode and a cathode, and asemi-permeable membrane impermeable to clay particles separating theanode and cathode, the anode and membrane forming an anodic compartmentin the cell, the cathode and membrane forming a cathodic compartment inthe cell; directing the particle suspension into the anode compartment;circulating an electrolyte solution having a pH range from about 2 toabout 7 through the cathode compartment; applying a direct current fieldbetween the anode and cathode to cause the solid particles to bedeposited from the suspension by electrophoretic action onto the anodeconcomitantly with the aqueous portion of the suspension being caused tomigrate electroosmotically through the semi-permeable membrane and intothe cathode compartment; and removing the solid particles from the anodeand the water from the cathode compartment.
 2. The method of claim 1wherein the electrolyte is 0.1% H₂ SO₄.
 3. The method of claim 1 whereina current density of about 0.05 to about 0.35 amperes per square inch ofeffective electrode area is delivered to the cell.
 4. The method ofclaim 1 wherein the spacing between the anode and membrane is about 1inch to about 11/2 inches.
 5. The method of claim 1 wherein the spacingbetween the cathode and membrane is about 1/16 inch to about 1/2 inch.6. The method of claim 1 wherein the membrane is a porous fabric havinga porosity of 0.5 to 4.0 cubic feet per minute.
 7. The method of claim 1wherein the aqueous suspension contains greater than 20% clay solids. 8.The method of claim 7 wherein the suspension contains 40%-60% claysolids.
 9. The method of claim 1 wherein the electrolyte solution is anacid solution ranging from about 0.1% to 1%.